There are several different methods of producing a reflective and transmissive display. The most well known and widely used method uses liquid crystal molecules as the electro-optic material. In the liquid crystal family, a vast range of molecules could potentially be used to create the electro-optic modulated material. Some of these liquid crystal molecules include, but are not limited to, twisted nematic, cholesteric-nematic, dichroic dye (or guest-host), dynamic scattering mode, and polymer dispersed molecules. Most of these liquid crystal molecules require other films, such as alignment layers, polarizers, and reflective films.
Another type of reflective display composing an electro-optic material is an electrophoretic display. Early work such as that described in U.S. Pat. No. 3,767,392, “ELECTROPHORETIC LIGHT IMAGE REPRODUCTION PROCESS”, used a suspension of small charged particles in a liquid solution. The suspension is sandwiched between two glass plates with electrodes on the glass plates. If the particles have the same density as the liquid solution then they will not be effected by gravity, therefore the only way to move the particles is using an electric field. By applying a potential to the electrodes, the charged particles are forced to move in the suspension to one of the contacts. The opposite charge moves the particles to the other contact. Once the particles are moved to one of the contacts they reside at that point until they are moved by another electric field, therefore the particles are bistable. The electrophoretic suspension is designed such that the particles are a different color than the liquid solution. Therefore, moving the particles from one surface to the other will change the color of the display. One potential problem with this display is the agglomeration of the small charged particles when the display is erased, i.e., as the pixel is erased, the particles are removed from the contact in groups rather than individually. Microencapsulating the electrophoretic suspension in small spheres solves this problem, as shown in U.S. Pat. No. 5,961,804, “MICROENCAPSULATED ELECTROPHORETIC DISPLAY”. FIG. 1 shows the typical operation of a microencapsulated electrophoretic display. In this display the particles are positively charged and are attracted to the negative terminal of the display by applying a voltage 7 across the electrophoretic material 37. The charged particles are white and the liquid solution they are suspended in is dark, therefore contrast in the display is optionally achieved by selectively moving some of the particles from one contact 5 to the other 5. In this type of display, the electro-optic material is the electrophoretic material and any casing used to contain the electrophoretic material.
A similar type of electro-optic display, a twisting ball display or Gyricon display, was invented by N. Sheridon at Xerox, and is shown in U.S. Pat. No. 4,126,854, “TWISTING BALL DISPLAY”. It was initially called a twisting ball display because it is composed of small spheres, one side coated black, the other white, sandwiched between two electroded 5 glass plates. Upon applying an electric field 7, the spheres with a positive charged white half and relative negative charged black half are optionally addressed (rotated), as shown in FIG. 2. Once the particles are rotated they stay in that position until an opposite field is applied. This bistable operation requires no electrical power to maintain an image. A follow on patent, U.S. Pat. No. 5,739,801, disclosed a multithreshold addressable twisting ball display. In this type of display, the electro-optic material is the bichromal spheres and any medium they may reside in to lower their friction in order to rotate.
Most electro-optic displays have problems with addressing the display. Since most of the electro-optic materials do not have a voltage threshold, displays fabricated with the materials have to be individually addressed. Some of the liquid crystal materials use an active transistor back plane to address the displays, but these type of displays are presently limited in size due to the complicated manufacturing process. Transmissive displays using liquid crystal materials and a plasma addressed back plane have been demonstrated in U.S. Pat. No. 4,896,149 and are shown in FIG. 3. A pair of parallel electrodes 36 are deposited in each of the channels 35, and a very thin glass microsheet 33 forms the top of the channels. Channels 35 are defined by ribs 34, which are typically formed by screen printing or sand blasting. A liquid crystal layer 32 on top of the microsheet 33 is the optically active portion of the display. A cover sheet 30 with transparent conducting electrodes 31 running perpendicular to the plasma channels 35 lies on top of the liquid crystal 32. Conventional polarizers, color filters, and backlights, like those found in other liquid crystal displays, are also commonly used. Displays fabricated using the plasma addressed back plane shown in FIG. 3 are also limited in size due to availability of the thin microsheet 33. One potential solution for producing large size displays is to use fibers to create the plasma cells as shown in FIG. 4. Using tubes to create a plasma cell was first disclosed in U.S. Pat. No. 3,964,050, and using fibers with wire electrodes to create the column driving plane in a transmissive plasma addressed liquid crystal display was disclosed in U.S. Pat. No. 5,984,747.
All of the above mentioned prior art focuses on creating a single display viewable on the surface of the panel. Therefore, there is a need in the art for a structure that can be used to create two independent images on both surfaces of a display panel.